U.S. Pat. No. 3,759,323 issued to Harry J. Dawson et al describes an important prior art primary surface heat exchanger for use as a recuperator core of a gas turbine. Dawson et al discloses a heat exchanger core made from a multiplicity of thin metal sheets which have been individually corrugated, or folded in a wavy pattern according to the method described in U.S. Pat. No. 3,892,119 issued to Miller et al. A large number of these metal plates are stacked on top of each other and the edges of the sheets are crushed to form the flat sections necessary to encase the assembly and to allow the attachment of suitable manifolding for conveying hot and cold fluids. The necessity for crushing the edges results in blockage, and hence the restriction of fluid flow. In addition, the depth of the corrugations in the unit described by Dawson et al is limited by the need to crush the edges. Deep corrugations or pleats cannot be crushed in an organized or predictable manner without causing severe blockage.
The pattern of the corrugations and the relatively thick crushed edges of the individual sheets described by Dawson et al result in rigidity in all directions in the heat exchanger unit. This may lead to the development of high thermal strains, especially when transient loads are characterized by steep gradients.
A major drawback of the prior art heat exchanger construction has been the presence of high stress concentration factors which have resulted from the need to crush the edges. In some applications the effect has been that of producing a multiplicity of cracks. Another problem has been the failure of the weld to penetrate at certain junctions, which results in a preformed crack. While such stress concentration factors may not be significant when the assembly is preloaded in compression, as intended, and when the transients are not steep, high stresses which lead to premature failures may appear under severe operating conditions and after prolonged periods of operation during which the preload is likely to be relaxed.